Use of a gaseous analgesic, such as nitrous oxide, in dental work of a potentially painful nature has become widely accepted. Nitrous oxide reduces the patient's sensitivity to pain without rendering the patient unconscious. It can be administered nasally, leaving the mouth unobstructed and free for dental work.
Commonly, nitrous oxide is administered using a nasal inhaler or nosepiece, placed over the patient's nose and connected through appropriate valves to a source of nitrous oxide and oxygen. When the patient inhales, only a small amount of nitrous oxide is absorbed by the lungs; when the patient exhales, most of the nitrous oxide is expelled. The small amount of gas absorbed during analgesia is released shortly after the flow of nitrous oxide terminates. Therefore, substantially all the gas delivered to the patient is ultimately exhaled by the patient. In many previous systems, this gas was simply released into the dental operatory.
Release of nitrous oxide to the environment is undesireable for two reasons. First, there is a tendency for the dentist and other dental office personnel to become anesthetized. Second, exposure to nitrous oxide is an occupational hazard; i.e., studies have shown a probable correlation between routine long-term exposure to nitrous oxide and certain serious diseases. Researchers suspect that the increased rate of spontaneous abortion among female anesthetists, increased incidence of birth defects among children born to anesthetists, and higher rates of disease of the liver and kidney, are related to chronic exposure to waste nitrous oxide. Clearly, a system for administering nitrous oxide without loss of gas to the environment is desirable.
In hospitals, the need to remove or prevent the release of anesthesia in the medical operating room has long been recognized. An anesthetic exhaust system intended to fit existing hospital equipment is described in U.S. Pat. No. 3,721,239 to Myers, wherein a manifold is placed around a conventional pop off valve to vent escaping anesthetic gas to a remote central vacuum.
Recently, a gas scavenging system for the dental operatory has been introduced, as described in U.S. Pat. No. 4,015,595 to Brown. The Brown system includes a first nosepiece disposed within a second nosepiece and a one way pressure relief valve for conducting gas from the inner to the outer nosepiece. Nitrous oxide is supplied to the inner nosepiece, while a vacuum source is connected to the region between the first and second nosepieces. In the Brown circuit, the analgesic effect varies inversely with the scavenging effect: as the vacuum flow is increased, the level of analgesic effect is reduced. The system is particularly sensitive to fluctuations in vacuum and is therefore difficult to adjust to a proper balance between analgesic effect and scavenging effect.
Another type of gas administration system is the Allen circuit, made by Dupaco, San Marcos, Calif. The Allen circuit has a pressure operated, one way exhalation valve that senses the increased pressure when the patient exhales. When the nosepiece pressure rises above a predetermined level the exhaust valve opens to remove gas. Pressurized gas inside the nosepiece tends to lift the nosepiece away from the face, causing gas leaks. The Allen circuit uses no edge scavenging means and relies on a snug fit between the mask and the patient's face to prevent the loss of gas.
In the above described system, there is no provision for air intake in the event that the gas supply system fails or where the patient requires a high inspiratory demand. Thus, a higher gas flow is necessary, resulting in greater use of nitrous oxide.